Digital cameras include image sensors to capture digital images and are generally operated in still image mode or live view mode. Different techniques may be used to produce still images and live view images. While it is important to produce still images with a high quality, images for live view or preview may be generated with less but still acceptable image quality.
Image sensors typically detect a light intensity per pixel. To allow color information to be recorded, color filters may be bonded to the substrate of the image sensor which allocate a certain color to a certain pixel, and each pixel detects the light intensity for the specific color. A typical pattern for a color filter used in digital cameras is a Bayer filter. A Bayer filter contains alternating rows of red and green filters and blue and green filters, wherein each row contains alternating red and green filters and blue and green filters, respectively. Green filters preferentially allow green light photons to pass to the detector (e.g., a photodiode) of the respective pixel of the image sensor. At the same time, red and blue light photons that arrive at the green filter are not transmitted by the filter and, therefore, not detected by the respective detector. Similarly, red filters preferentially allow red light photons and blue filters allow only blue light photons to pass to the respective detector.
Each image frame has an image pixel resolution that depends on a pixel resolution of the image sensor and generally describes the detail of information that a digital image contains. In other words, by increasing the image pixel resolution, the level of detail in the image can be increased. The pixel resolution of an image frame is typically described by a number of pixel columns (image width) and a number of pixel rows (image height) which result in a number of total pixels that need to be processed by the specialized image processor.
When a Bayer filter is applied in front of an image sensor, the resulting image frame produced by the image sensor needs to be interpolated or demosaiced to generate all three colors for each pixel so that the digital image can be displayed, printed or stored.
In digital cameras, interpolating or demosaicing may be performed by specialized image signal processors (ISPs) but also by general purpose processors (CPUs) which execute image processing software programs.
The process of interpolating or demosaicing can be very complex depending on the effectiveness of the mathematical algorithms used in particular because of the much higher number of pixels that need to be processed in still image mode. U.S. Pat. No. 6,292,218 B1 describes an electronic camera in which the more complex digital techniques for generating a high quality still image are implemented in software and executed on a CPU, whereas the less complex techniques for producing preview images of acceptable quality are implemented in an application specific integrated circuit (ASIC).
However, utilizing a CPU for still image processing may significantly increase the cost of manufacturing high resolution digital cameras. On the other hand, processing still images with ISPs that have correspondingly high processing capabilities may also result in significantly higher manufacturing costs for the digital camera. Therefore, new approaches are needed which allow the use of ISPs with relatively low processing capabilities to also process full resolution high quality still image frames without the need of a CPU for image processing, such as interpolating and demosaicing.